1. Field of the Invention
The present invention relates to a photoelectric converting apparatus having a semiconductor transistor for accumulating carriers generated by light excitation into a control electrode region.
2. Related Background Art
FIG. 1A is a schematic cross-sectional view of a photoelectric converting apparatus as disclosed in European Patent Application Laid-Open No. 0201270. FIG. 1B is an equivalent circuit diagram of this apparatus.
In the diagrams, each photosensor cell is electrically separated from the adjacent photosensor cell by an n.sup.+ element separating region 6.
Each photosensor cell has the following structure.
An n.sup.- region 4 of a low n-type impurity concentration is formed by an epitaxial technique or the like. A p base region 9 is then formed in the n.sup.- region 4 by doping impurities of the p-type. An n.sup.+ emitter region 15 is formed in the p base region 9 by doping n type impurity at a high concentration according to an impurity diffusion technique, ion implantation technique, or the like. That is, a bipolar transistor of npn type is formed.
A polysilicon layer 14 having a predetermined area is formed over the n.sup.- region 4 through an oxide film 12. The polysilicon region 14 faces the p base region 9 through the oxide film 12. The polysilicon region 14, the oxide film 12 and the p base region 9 construct a capacitor C.sub.ox to control the potential of the p base region 9.
In addition, an emitter electrode 19, a capacitor electrode 17 electrically connected to the polysilicon region 14, an n.sup.+ region 2 having a high impurity concentration on the back surface of the n type silicon substrate 1, a collector electrode 21, and the like are formed, respectively.
The fundamental operation of the photoelectric converting device as shown in FIGS. 1A and 1B will now be described. The light enters and is absorbed by the p base region 9 of the bipolar transistor. The charges (holes in this case) corresponding to the incident light amount are accumulated in the p base region 9. The voltage of p base region 9 (base potential) changes in accordance with the charges accumulated (accumulating operation).
Next, the base potential is increased by applying a positive voltage to the capacitor electrode 17. The p base region 9 is forwardly biased for the n.sup.+ emitter region 15. The voltage of the p base region 9 is read out of the emitter electrode 19 which is in the floating state by the operation of the transistor which is executed by the forward biasing operation. Thus, an electric signal corresponding to the incident light quantity can be derived (reading operation).
By connecting emitter electrode 19 to the ground potential and then applying a pulse of a positive voltage to the capacitor electrode 17, the charges accumulated to the p base region 9 are eliminated. Thus, the p base region 9 is forwardly biased for the n.sup.+ emitter region 15 and the accumulated charges are eliminated (refreshing operation). Thereafter, the accumulating, reading, and refreshing operations are repeated.
The capacitor C.sub.ox may be omitted in some instances.
In the foregoing photoelectric converting apparatus, it is desirable that a current amplification factor, H.sub.fe, under the emitter grounded condition is as high as possible in order to raise the driving capability, to accomplish the high reading speed and to reduce the breakdown of the holes in the p base region during the reading operation. To maintain an insulating breakdown voltage and to assure the high H.sub.fe, the impurity concentration of the p base region 9 needs to be suppressed as low as about a value within a range from 1.times.10.sup.15 to 5.times.10.sup.17 cm.sup.-3.
However, the reduction of the impurity concentration of p base region 9 causes the following problems.
(1) The impurity concentration at the interface between the p base region 9 and the oxide film 12 is further decreased due to the segregation. Additionally, as a result, in a device having this construction a surface generating current I.sub.surf is generated, causing a large noise component.
(2) When a pulse of a positive voltage is applied to the capacitor electrode 17, the polarity of the p base region 9 just under the polysilicon region 14 is inverted into the n type and a capacitance value of the capacitor C.sub.ox decreases, so that the output decreases.
(3) When a surface current is generated at the interface between the p base region 9 and the oxide film 12, a channel is formed on the surface of the p base region 9, so that punchthrough easily occurs on the surface between the collector and the emitter.